Modern oil field operations require that the borehole be made accessible to a variety of downhole tools. Operations requiring borehole access include fluid sampling, formation pressure testing, and logging. Logging can be performed by several methods including wireline logging, “logging while drilling” (LWD), and through-the-bit logging.
In wireline logging, a probe or “sonde” is lowered into the borehole after some or the entire well has been drilled and the drillstring extracted. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.
Through-the-bit logging involves introducing a logging instrument into the borehole through a port located in the drill bit. The logging instrument (potentially a wireline tool) is lowered or pumped into the borehole through the interior passage of the drill string. At the lower end of the drill string, a port in the drill bit allows the logging instrument to pass into the borehole. Wireline tools may be used to perform logging below the drill bit, or the logging tool may be suspended from the drill string. For example, logging may be performed as the drill string is removed from the borehole (“logging while tripping”). Through-the-bit logging allows examination of the borehole in a relatively benign environment without first extracting the drillstring from the borehole, and accordingly may offer potential advantages over both wireline logging and LWD. Avoiding the harsh drilling environment of LWD potentially results in improved data quality, and a decrease in tool failures and the associated costs. The ability to log the formation when desired, without needlessly tripping the drillstring out of the hole, may result in substantial time savings when compared to conventional wireline logging.
Conventional drilling operations employ drill bits with nozzles that spray drilling fluid at high pressure to clear cuttings from the bit and from the bottom of the hole. The nozzles may not be large enough to serve as a tool port, whereas a tool port of sufficient size for through-the-bit logging may prevent effective clearing of cuttings.
The drawings show illustrative embodiments that will be described in detail. However, the description and accompanying drawings are not intended to limit the claimed invention to the illustrative embodiments, but to the contrary, the intention is to disclose and protect all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.